The invention relates generally to measuring the temperature of a gas, and more particularly to a method and system for generating a series of laser-induced fluorescence peaks where each peak has contributions from an ensemble of populated energy states representing the rotational level distribution of multiple vibrational levels of the ground electronic state of a molecule, and where ratios of the peaks are used as a means to instantaneously determine the temperature of a low density gas such as that found in the wake of a structure in a high-speed gaseous flow.
Understanding material or structural response to heat is important or critical in many applications. One area of particular interest is the study of aerothermodynamics which attempts to evaluate and predict heat response of materials or structures used in aeronautics and aerospace applications. Such applications include analysis of aerothermodynamic problems such as aerobraking, dynamics of planetary probes with payloads, and transitional flows impinging on thermal protection systems. For proper analysis, it is necessary to obtain data on flow unsteadiness, mixing, separation and reattachment points, along with near-wake and shear layer features such as temperature, density and transition location. This requires diagnostics capable of measurements in the low-density wake region created by models in hypersonic flows. For years, data has been obtained using intrusive flow-field-perturbing techniques such as Pitot tubes, thermocouples and hot-wire anemometry. However, use of such flow-disturbing devices is undesirable when trying to isolate/evaluate model performance.
More recently, a few molecular-based optical methods for making various diagnostic measurements have been developed. With respect to temperature measurements or thermometry as it is known, the optical methods generally involve some form of laser-induced fluorescence (LIF) in which one or more lasers are used to xe2x80x9cinterrogatexe2x80x9d individual rotational and vibrational states of a naturally-occurring or molecular-seeded gas. Temperature is measured by probing the distribution of population over two or more states. Currently, there are three main approaches to LIF thermometry: i) excitation scans, ii) two-line interrogation, and iii) thermally-assisted LIF, each of which will be described briefly below.
Using a single continuously-tunable laser, the excitation scan method changes laser wavelength to scan over several rotational levels in a single specified vibrational level of the ground electronic state. Fluorescence from each transition is recorded with the fluorescence being proportional to the population of the absorbing rotational level. Temperature is determined by fitting the fluorescence excitation spectrum to the Boltzmann equation. However, the time required to scan multiple molecular transitions makes this approach unsuitable for making instantaneous temperature measurements. Further, the complexities associated with changing the laser""s wavelength add to the cost of the measurement system.
Two-line fluorescence thermometry requires the use of two lasers fired sequentially into a gas. Each laser is typically tuned to a different rotational level in a single specified vibrational level of the ground electronic state. Using a fluorescence excitation spectrum, the relative population of the two states is measured and temperature is determined using the Boltzmann equation. However, the use of two lasers that must be fired sequentially adds to the cost and complexity of the system. Further, since the lasers are fired sequentially, this approach requires more than twice the time to measure temperature as a single laser approach.
Thermally-assisted LIF thermometry uses a single laser to pump a single rotational level from the ground electronic state to an excited electronic state. If the correct conditions exist, collisions with other gas molecules rapidly distribute the population in the excited electronic state. A model of the various collision transfer processes is used to predict the resulting population distribution as a function of temperature. Since the fluorescence spectrum reflects the excited state distribution, the fluorescence spectrum can be used as an indication of temperature. Thermally-assisted LIF assumes that gas composition and the temperature dependence of all state-specific collision transfer rates are known. However, since these quantities are generally not known, assumptions must be made which affect accuracy of the thermometry.
Accordingly, it is an object of the present invention to provide a method and system for determining temperature of a low-density gas.
Another object of the present invention is to provide a non-intrusive method and system for determining temperature of a low-density gas.
Still another object of the present invention is to provide a method and system for determining the temperature of a gas in the low-density wake region of an object subjected to a high-speed gaseous flow.
Yet another object of the present invention is to provide a method and system for determining the temperature of a low-density gas based on laser-induced fluorescence.
A further object of the present invention is to provide an improved laser-induced fluorescence thermometry methodology and system.
A still further object of the present invention is to provide a laser-induced fluorescence thermometry methodology and system that can provide a nearly instantaneous temperature determination.
Another object of the present invention is to provide a laser-induced fluorescence thermometry method and system that minimizes the use of toxic and/or corrosive seed chemicals.
Still another object of the present invention is to provide an instantaneous method and system for determining temperature of a gas by sampling the rotational level distribution from multiple populated vibrational levels of the ground electronic state.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method and system are provided for determining temperature of a low density gas defined by a density not to exceed approximately 3xc3x971017 molecules per cubic centimeter. Such a low density gas environment is seeded with molecules of iodine wherein a seeded environment is created. The Cordes bands of the iodine molecules are excited with light energy of a known wavelength (e.g., between 175-210 nanometers) to generate fluorescent emission having light intensity. A fraction of the light intensity is collected and passed through a spectrograph. The spectrograph separates the light into its component wavelengths. The result is a series of peaks of light intensity versus wavelength. The light intensity in a given peak can be related to population in a specific vibrational energy level of the ground electronic state. After being assigned, appropriate peaks are then fit to a vibrational Boltzmann distribution. More specifically, one of the vibrational energy level peaks associated with molecule population in a ground vibrational energy level at a wavelength greater than the known wavelength of the light energy is selected. Next, a plurality of ratios are generated that define a unique linear relationship for a temperature of the seeded environment. Each of these ratios is defined by a ratio of the selected one of the vibrational energy level peaks (in a ground vibrational energy level) to another unique peak (of the vibrational energy level peaks) associated with molecule population in a vibrational energy level that is greater than the ground vibrational energy level.